Method for centring and dimensioning an image on a cathode-ray tube

ABSTRACT

The steps of the process comprise:  
     (a) measuring ( 50 ) the durations (T1 HAV , T2 HAV ) of the vertical black edges of the image and modifying step by step the adjustment (HPOS) for horizontal centering to obtain equal lateral vertical edges;  
     (b) measuring ( 52 ) the durations (T1 HAV  and T2 HAV ) of the vertical black edges of the image to calculate the adjustment (HSIZE) of the horizontal dimension of the image so as to cause the vertical black edges to disappear;  
     (c) measuring ( 54 ) the durations (T1 VAV  and T2 VAV ) of the horizontal black edges of the image to calculate the adjustment of the vertical dimension of the image (VSIZE′) and the adjustment of the vertical centering of the image (VPOS′) so as to cause the horizontal black edges to disappear and to center the image vertically, and  
     (d) recording ( 56 ) the adjustment values obtained (HPOS, HSIZE, VSIZE′ and VPOS′) in a memory of the display calculator.

[0001] The present invention relates to cathode ray tubes used for displaying images on television (TV) sets and personal computers (PC). More particularly, it relates to a method of automatically centering and dimensioning a displayed image.

[0002] A cathode ray tube for displaying images on a screen formed by its faceplate generally comprises electronic circuits which control the scanning of the screen by an electron beam so as to activate or not the luminescence of screen pixels and thus produce the desired image.

[0003] These electronic scanning circuits are driven via a display controller through electrical signals which can have different sources, such as computer signals, signals from laser disks or game consoles. Because of their diversity, a same setting for the size and position of the image cannot be suitable for all sources and can result in a bad centering of the image on screen and a distorted image. These defects can moreover exist for a cathode ray tube from the factory in the case where the settings are not properly adjusted.

[0004] This can account for the presence of black stripes on the vertical or horizontal edges of the image, a horizontal or vertical shift of the image, or an image distortion in the horizontal or vertical direction.

[0005] In the state of the art, these faults are corrected manually by the user through control buttons which bring up adjustment menus and sub-menus. Such an operating procedure is neither fast nor simple, notably owing to the fact that there are only few control buttons, which requires the user to employ a same button for several different functions.

[0006] This is all the more inconvenient as these adjustments must be made as a function of the screen's operating mode, for example to pass from one video mode to another or from a classical 640×480 pixel screen to a higher definition 1280×1024 pixel screen.

[0007] Accordingly, when changing from one video mode to another, the display calculator analyzes the new horizontal and vertical synchronization signals, calculates their frequencies and carries out the necessary adjustments to display a new image in the new mode. However, the image obtained is never perfectly adapted to the screen size, and consequently suffers from faults as regards centering and dimensioning or size mentioned above.

[0008] Therefore, in the state of the art, the user must activate the control buttons provided for that purpose until the desired image is obtained, and the adjustments made are written into a memory of the display calculator, not only for the current session, but also for later sessions with the same display mode.

[0009] However, despite this memory storage of the settings, the user often needs to readjust the latter during a subsequent use of the same display mode, and all the more so as the settings of the mode entered in memory are not always adapted to all software which use that mode.

[0010] An object of the present invention is thus to implement a process for automatically centering and dimensioning an image on the screen of a cathode ray tube.

[0011] The invention relates to a method of centering and dimensioning an image on a cathode ray tube whose display signals are supplied by a display calculator, the method being characterized in that it comprises the following steps:

[0012] (a) measuring the durations of the vertical black edges of the image and modifying step by step the adjustment (HPOS) for horizontal centering to obtain equal lateral vertical edges;

[0013] (b) measuring the durations of the vertical black edges of the image to calculate the adjustment (HSIZE) of the horizontal dimension of the image so as to cause the vertical black edges to disappear;

[0014] (c) measuring the durations of the horizontal black edges of the image to calculate the adjustment of the vertical dimension of the image (VSIZE′) and the adjustment of the vertical centering of the image (VPOS′) so as to cause the horizontal black edges to disappear and to center the image vertically, and

[0015] (d) recording the adjustment values obtained (HPOS, HSIZE, VSIZE′0 and VPOS′) in a memory of the display calculator.

[0016] Steps (a), (b) and (c) can be performed in any order because they are independent of each other, but it is advisable to perform step (a) before step (b), given that the precision for the calculation of the setting (HSIZE) for the horizontal dimension of the image depends on the perfect horizontal centering of the image.

[0017] Step (d) can come into play after each step (a), (b) or (c) to record the value of the setting obtained by the step having just been finished.

[0018] The method is implemented only if the image is sufficiently stable, this being detected by checking that the positions of the vertical and horizontal edges have fluctuations below a certain threshold. This stability is checked before each step (a), (b) or (c)

[0019] Other characteristics and advantages of the present invention shall become apparent from reading the following description of a preferred embodiment, in relation with appended drawings in which:

[0020]FIG. 1-A shows an image on a screen of a cathode ray tube which is not centered and exhibits a black surround, and FIG. 1-B shows the same image after implementation of the process in accordance with the invention;

[0021]FIG. 2 is a diagram showing the relations between the horizontal position of the image on screen and the horizontal scanning signal for an image line;

[0022]FIG. 3 is a diagram analogous to that of FIG. 2, but showing the relations between the vertical position of the image on screen and the vertical scanning signal for a complete image;

[0023]FIG. 4 is a diagram showing the main steps of the process in accordance with the invention;

[0024] FIGS. 5-A and 5-B show the steps in the horizontal image centering algorithm in accordance with the process of the invention, this algorithm being preceded by an algorithm for checking the image stability;

[0025]FIG. 6 is a curve showing the variation of H_(AMPMIN) as a function of the horizontal scanning frequency for a given range of frequencies; and

[0026]FIG. 7 is a diagram showing the adjustment for vertical centering and vertical dimensioning.

[0027]FIG. 1-A shows the faceplate 10 of cathode ray tube 12, on the screen of which appears an image 14 whose vertical edges 16 and horizontal edges 18 are black (i.e. dark), so indicating that the image 14 is not centered at the center of the screen and that it only occupies a part of the screen.

[0028] As indicated in the introductory portion above, the adjustments for centering and dimensioning the image are at present made by the user through buttons 20 which bring down menus and sub-menus on the screen to guide the user in the adjustments.

[0029] These control buttons 20 are active for the adjustments via a display calculator which supplies the values of horizontal and vertical scanning signals. This display calculator is capable of receiving the video signals and analyzing them to output these scanning signals.

[0030] In accordance with the invention, a control button 22 (FIG. 1-B) is added to implement the inventive process and obtain in a few seconds the correctly centered and dimensioned image of FIG. 1-B.

[0031] The process of the invention is based on the measurement of the length, in units of time, of vertical and horizontal black edges, these measurements then serving for carrying out algorithmic operations and calculations leading to a modification of the image centering and its dimensions.

[0032]FIG. 2 shows the image 14 and the corresponding horizontal scanning signal 30 as a function of time t for a line of the image, i.e. the current I_(H) flowing in the horizontal deflection coil (yoke). The figure also shows the horizontal synchronization pulses 32 and 34 (HFBACK) which determine the start and end points of a horizontal scanning signal, the start of horizontal scanning corresponding to the falling edge of pulse 32 and the end corresponding to the rising edge of pulse 34. The duration of the scanning return (flyback) is given by the duration of pulse 32 or 34.

[0033] When the image exhibits vertical black edge portions, this comes from the fact that signals of the Red, Green and Blue components at the start and end of horizontal scanning are all below a certain level. The measure of the time duration T1_(HAV) between the falling edge and the start of the left of the image indicates the extent of the left vertical black edge portion while a measure of T2_(HAV) between the end of the right of the image and the rising edge indicates the extent of the right vertical black edge portion.

[0034] It then follows that if T1_(HAV)=T² _(HAV), then the image is centered horizontally, whereas it is not centered if T1_(HAV) is different from T2_(HAV).

[0035] The process in accordance with the invention obtains horizontal centering of the image by:

[0036] measuring T1_(HAV) and T2_(HAV) in a repetitive manner,

[0037] comparing T1_(HAV) and T2_(HAV) at each time,

[0038] displacing the image by one unit towards:

[0039] the right if T1_(HAV)<T2_(HAV),

[0040] the left if T1_(HAV)>T2_(HAV) until is obtained the equality T1_(HAV)=T2_(HAV).

[0041] The measurement of T1_(HAV) and T2_(HAV) is performed by the display calculator using a device provided to that effect and known per se.

[0042] TI_(HAV) and T2_(HAV) do not allow to obtain the horizontal dimensioning of the image for making the vertical black edge portions disappear, since the time interval between two horizontal synchronization pulses 32 and 34 is fixed, irrespective of the horizontal width of the image. The process of the invention produces this horizontal dimensioning by modifying the amplitude of the curve 30 in accordance a formula, as shall be described below.

[0043]FIG. 3 shows the image 14 and the corresponding vertical scanning signal 40 as a function of time t for a complete image, i.e. the voltage V_(v) of the vertical deflection sawtooth signal for line-by-line vertical screen scanning. The figure also shows the vertical synchronization signals 42 and 44 (VFBACK) which determine the start and end points of a vertical scanning signal, the duration of the pulse determining the duration of the return period for the vertical scanning signal.

[0044] As in the case of horizontal line scanning, the time periods T1_(VAV) and T2_(VAV) respectively define the extents of the top black edge portion and the bottom black edge portion of the image. However, these time periods cannot serve to center the image vertically because the time interval between the top and bottom edges of the image and the corresponding pulses 42 and 44 remain constant irrespective of the vertical position of the image.

[0045] Likewise, the time periods T1_(VAV) and T2_(VAV) cannot serve directly for vertically dimensioning the image because the time period of the vertical synchronisation pulses 42, 44 remains the same irrespective of the image height. The measurement of T1_(VAV) and T2_(VAV) is carried out by the display calculator using the above-mentioned measuring device for measuring T1_(HAV) and T2_(HAV).

[0046] The process according to the invention provides the vertical centering and the vertical dimensioning by modifying the amplitude of the curve 40 in accordance with a formula as shall be described hereafter.

[0047] The diagram of FIG. 4 illustrates the main steps of the invention, which comprises the steps of:

[0048] (a) measuring T1_(HAV) and T2_(HAV) to calculate the adjustment HPOS to perform in order to obtain the horizontal centering of the image (box 50),

[0049] (b) measuring T1_(HAV) and T2_(HAV) to calculate the adjustment to perform HSIZE in order to obtain the horizontal dimensioning of the image (box 52),

[0050] (c) measuring T1_(VAV) and T2_(VAV) to calculate the adjustment to perform VPOS and VSIZE in order to obtain at the same time vertical centering and the vertical dimensioning of the image (box 54), and

[0051] (d) recording the values HPOS, HSIZE, VPOS′ and VSIZE′ in a memory (box 56) of the display calculator.

[0052] If an error arises during one or another of steps 50, 52 and 54, notably in the case of image instability, the starting values are restored in the memory (box 58). These errors can arise from an image which is unstable, which is shifting, which is too small to be adjusted, or for any other reason.

[0053] Note that steps (a), (b) and (c) can be performed in any order, but it appears logical to start with the simplest, which is the horizontal centring step, owing to the fact that it stems directly from the measurement of T1_(HAV) and T2_(HAV). Moreover, step (b) yields more precise results if it follows from step (a).

[0054] The diagram of FIGS. 5-A and 5-B shows in detail the operations to be performed during step (a) for horizontal centring. However, the first operations 60, 62, 64, 66, 68 and 70 are repeated, wholly or in part, at the start of each step (a), (b) or (c) to check that the image is stable within the established limits. These first operations comprise the steps of:

[0055] pressing on button 22 (arrow 60) to trigger off the operations,

[0056] performing a first series of measurements to obtain a first set of pairs of values T1_(HAV1) and T2_(HAV1), T1_(VAV1) and T2_(VAV1) (box 62),

[0057] performing a second series of measurements to obtain a second set of pairs of values T1_(HAV2) and T2_(HAV2), T1_(VAV2) and T2_(VAV2) (box 64),

[0058] subtracting the second set of pairs of values from the values from the first set (box 66) to obtain difference values DIFF in terms of absolute values,

[0059] comparing the difference values DIFF with a threshold TMUDIFF (lozenge 68),

[0060] stopping the operations if DIFF>TMUDIFF, for the image is then considered to be unstable or shifting, or passing onto the next operation (lozenge 70) in the opposite case.

[0061] Note that the series of measurements T1_(AV) and T2_(AV) which concern the horizontal deflection are preferably only performed just before each horizontal adjustment (a) or (b) to determine the horizontal stability of the image.

[0062] Likewise, the series of measurements T1_(VAV) and T2_(VAV), which concern the vertical deflection, are only performed just before the vertical adjustments, preferably for centering and dimensioning to determine the vertical image stability,

[0063] comparing T1_(HAV) and/or T2_(HAV) (lozenge 70) with a maximum value MAX and stopping the operations if it is reached, for the image is then considered to be too small and hence not exploitable, or that the video signal is bad (lozenge 70); in the case of a negative comparison, passing on to the next operation, the first concerning the horizontal centering proper, which comprises the steps of:

[0064] checking whether the negative comparison arrives for the first time or not (lozenge 72), and

[0065] in the case of a positive check, passing on to the next operation comprising the steps of:

[0066] comparing T1_(HAV) with T2_(HAV) (lozenge 74), and

[0067] stopping the horizontal centering operations in the case of an inequality, for the image is already horizontally centered, and passing on to step (b),

[0068] the image must be displaced to the right if T1_(HAV)<T2_(HAV), such an event being memorized by a flag at the 0 state,

[0069] the image must be displaced to the left if T1_(HAV)>T2_(HAV), such an event being memorized by the flag, but in this case at the 1 state,

[0070] in the case of a negative check, or in the case where the image must be displaced, passing on to the next operation.

[0071] The value 0 or 1 for the flag indicates the direction in which the image is to be displaced, the displacement being effected in a stepwise manner by incrementing or decrementing the centering adjustment value HPOS.

[0072] The following operations involve comparing T1_(HAV) with T2_(HAV) and modifying the centering adjustment value HPOS in the direction indicated by the value of the flag until detection of the equality T1_(HAV)=T2_(HAV). These operations are presented in the diagram of FIG. 5-B.

[0073] The first operation (box 80) consists in checking whether the flag is at logic 1, indicating that the image is off-centered in the right direction and must be brought back to the left.

[0074] if the check is positive, the following operation consists in checking whether T1_(HAV)>T2_(HAV) (lozenge 82), and there are three possible responses:

[0075] (i) T1_(HAV)=T2_(HAV), in which case the image is centered and the horizontal centering operations are stopped to pass on to step (b),

[0076] (ii) T1_(HAV)>T2_(HAV), in which case the image is off-centered in the right direction and must be displaced to the left by decrementing the adjustment value HPOS by one unit (box 86); moreover, a loop counter 90 is incremented by one unit;

[0077] (iii) T1_(HAV)<T2_(HAV), in which case the image which was off-centered in the right direction since the start of the operations is now off-centered towards the left, which means that the centering value HPOS has been exceeded by one unit. This overshoot is corrected by incrementing the horizontal adjustment value HPOS by one unit (box 94). With this incrementation, the value of HPOS corresponds to the center position, and the horizontal centering operations are stopped to pass on to step (b).

[0078] If the flag is not at logic 1, i.e. the image is off-centered to the left and must be brought back to the right, the following operation (box 84) consists in checking whether T1_(HAV)<T2_(HAV), and there are three possible solutions as in the previous case:

[0079] (i) T1_(HAV)=T2_(HAV), in which case the image is centered and the horizontal centering operations are stopped to pass on to step (b),

[0080] (ii) T1_(HAV)<T2_(HAV), in which case the image is off-centered in the left direction and must be displaced to the right by incrementing the adjustment value HPOS by one unit (box 88); moreover, a loop counter 90 is incremented by one unit;

[0081] (iii) T1_(HAV)>T2_(HAV), in which case the image which was off-centered in the left direction since the start of the operations is now off-centered towards the right, which means that the centering value HPOS has been exceeded by one unit. This overshoot is corrected by decrementing the horizontal adjustment value HPOS by one unit (box 96). With this incrementation, the value of HPOS corresponds to the center position and the horizontal centering operations are stopped to pass on to step (b).

[0082] If the loop counter 90 is incremented, this means that the centering value HPOS has not yet been obtained and that it is necessary start again all the operations described above (new loop) starting from step 62 consisting of measuring new values of T1_(HAV) and T2_(HAV) subsequent to the new value of HPOS.

[0083] However, this new loop is performed only if the number of loops has not exceeded a certain threshold BMAX. The operation consists in:

[0084] comparing (lozenge 92) the contents of the loop counter 90 with BMAX,

[0085] stopping the operations if the centering has not been achieved after a set number of shifts BMAX,

[0086] or starting a new loop if BMAX is not attained.

[0087] To set the horizontal dimension of the image such that it takes up the entire width of the screen, i.e. without vertical black edges, it is necessary to change the amplitude setting for the current flowing in the horizontal deflection coil, such an adjustment being represented by a value HSIZE which can vary e.g. between 0 and 255. It is this value HSIZE for obtaining a maximum image width which is calculated by the method according to the invention, this value being dependent on many parameters, and notably T1_(HAV) and T2_(HAV).

[0088] The formula which enables to calculate HSIZE is: ${HSIZE} = {\frac{{A_{Vopti}\left( {T^{\prime}/{Td}^{\quad \prime}} \right)} - H_{AMPMIN}}{H_{AMPMAX} - H_{AMPMIN}} \times {HSIZE}_{MAX}}$

[0089] In which formula:

[0090] A_(Vopti) is the optimum amplitude of the current in the horizontal deflection coil to obtain an image of optimum width; this amplitude is measured for a type of cathode ray tube and in a reference video mode,

[0091] HSIZE_(MAX) is the maximum value of HSIZE, e.g. 255 as indicated above,

[0092] H_(AMPMAX) is the maximum variation of the current in the horizontal deflection coil to obtain a maximum horizontal deflection; this value varies as a function of the horizontal scanning frequency f(fH) as described below;

[0093] H_(AMPMIN) is the minimum variation of the current in the horizontal deflection coil to obtain a minimum horizontal deflection; this value varies as a function of the horizontal scanning frequency f(fH) as described below;

[0094] T′ is the total duration of a horizontal line, i.e. the duration of the period T of the horizontal synchronization signal, from which are subtracted the duration of the flyback pulse T_(FLYBACK), in general three microseconds, and a duration of safety margins, e.g. 0.6 microseconds, and

−Td′=T−(T1_(HAV) +T2_(HAV) +T _(FLYBACK))

[0095]  i.e. the duration of the image on screen between these black edges.

[0096] This formula is established by supposing that the current varies linearly, which is not the case, so that to take into account the fact that the curve is S shaped, the coefficient applied A_(Vopti) must be replaced by

(1.8T′−Td′)/2.8Td′,

[0097] which coefficient can change depending on the type of cathode ray tube and its control device.

[0098] The values for H_(AMPMIN) and H_(AMPMAX) are determined by means of, curves as a function of the horizontal scanning frequency f(H), this being effected for frequency ranges.

[0099] For instance, curve 100 of FIG. 6 shows the function of H_(AMPMIN)=f(fH) for a range of frequencies from 34 kHz to 41 kHz for the case of a given cathode ray tube. The abscissa x is graduated in kHz while the ordinate is graduated in H_(AMP) x 10 mA. There is thus obtained a straight line whose equation is:

Y=−5.22x+1265.1=ax+b.

[0100] This equation is different for another range of frequencies.

[0101] Coefficients a and b determined for each range of frequencies are recorded in a memory so that they can be read in view of calculating H_(AMPMIN) according to the horizontal scanning frequency.

[0102] H_(AMPMAX) is obtained in the same manner as for H_(AMPMIN).

[0103] As a result, if there are eight frequency ranges, there shall be sixteen pairs of coefficients (a,b) which define the sixteen variation curves, eight for H_(AMPMIN) and eight for H_(AMPMAX).

[0104] To achieve vertical centering and vertical dimensioning, the method according to the invention consists in measuring the values T1_(VAV) and T2_(VAV) for the image which appears on the screen, and then first calculating VSIZE to obtain the vertical dimensioning and subsequently VPOS′ to obtain the vertical centering according to the following formulae: VSIZE^(′) = 0.5[(3VSIZEMAX + 2VSIZE)] ⋅ [(Td × T^(′))/(Td^(  ′) × T)] − 1.5(VSIZEMAX)  and

 VPOS′=VPOS+(A−B)

with

A=[(2.25+1.5.(VSIZE/VSIZEMAX)]×[(0.5−T1/T).(VPOSMAX/0.6)]

and

B=[(2.25+1.5.(VSIZE′/VSIZEMAX)]×[0.5−T1′/T′).(VPOSMAX/0.6)]

[0105] To define the parameters of these formulae, reference shall be made to FIG. 7, which shows the sawtooth for vertical scanning 40, but inversed with respect to that of FIG. 3. The abscissa shows the duration and the ordinate shows the voltage V_(OUT). On this sawtooth is placed the image 112 to be vertically centered and dimensioned and a reference image which is appropriately vertically centered and dimensioned.

[0106] In FIG. 7, T₁, T₂ correspond in time periods respectively to the start and end of the reference image 114, while T1′ and T2′ correspond in time periods respectively to the start and end of the image 112 to be centered and dimensioned. The following relations are then established:

[0107] The duration Td of the reference image is given by Td=T2−T1, and the duration Td′ of the image to be centered is given by Td′=T2′−T1′.

[0108] Also, T1=T1_(VAV) and T2=T−T2_(VAV), T being the total duration of a sawtooth. Similarly, T1′=T1′=_(VAV) and T2′=T′−T2′_(VAV).

[0109] The reference image 114 is obtained by a manual adjustment in a reference video mode on a given type of cathode ray tube and the values T1_(VAV) and T2_(VAV) are measured and entered into a memory to be used for the automatic adjustments on that type of cathode ray tube. The same applies for the value VSIZE, which corresponds to that reference image, while VZIZEMAX is the maximum adjustment value, for example 256.

[0110] These elements allow to calculate the value VSIZE′ according to the above formula, i.e. the adjustment value that will allow to obtain an image which is appropriately vertically dimensioned.

[0111] By knowing VSIZE′, it is possible to calculate VPOS′ according to the above formula, which also uses the value VPOSMAX, which is the maximum adjustment for the vertical centering.

[0112] The invention has been described in its application to the adjustment of a cathode ray tube by the user of a computer or a television set in which the cathode ray tube forms the display screen. The invention also applies to the implementation of the process for adjusting the horizontal and vertical deflection coils at the end of a cathode ray tube production line.

[0113] Indeed, at the end of a cathode ray tube production line, the image generated to test for the correct operation of the cathode ray tube exhibits faults which an operator corrects in various ways. One of the faults concerns a bad alignment between the image and screen centers and, to correct it, the operator first performs image centering and dimensioning adjustments using the buttons 20 (FIG. 1-A) and then the adjustments in the electronic and magnetic circuits (deflection coils) to displace the image center and make it coincide with the center of the screen. In this sequence of operations, the method of the invention can be implemented to obtain the centering HPOS and HPOS′, and possibly the dimensioning HSIZE and VSIZE′.

[0114] For this adjustment, the operations to be performed would then be as follows:

[0115] display a calibrated image, for example a white image with a perfectly centered cross,

[0116] launch the process of the invention wholly or in part,

[0117] modify the electronic and magnetic settings for the screen to bring the cross to the center of the screen. 

1. Method of centering and dimensioning an image on a cathode ray tube whose display signals are supplied by a display calculator, said method being characterized in that in that it comprises the following steps: (a) measuring the durations (T1_(HAV), T2_(HAV)) of the vertical black edges of the image and modifying step by step the adjustment (HPOS) for horizontal centering to obtain equal lateral vertical edges; (b) measuring the durations (T1_(HAV) and T2_(HAV)) of the vertical black edges of the image to calculate the adjustment (HSIZE) of the horizontal dimension of the image so as to cause the vertical black edges to disappear; (c) measuring the durations (T1_(VAV) and T2_(VAV)) of the horizontal black edges of the image to calculate the adjustment of the vertical dimension of the image (VSIZE′) and the adjustment of the vertical centering of the image (VPOS′) so as to cause the horizontal black edges to disappear and to center the image vertically, and (d) recording the adjustment values obtained (HPOS, HSIZE, VSIZE′ and VPOS′) in a memory of the display calculator.
 2. Method according to claim 1, characterized in that steps (a), (b) and (c) can be performed in an arbitrary order.
 3. Method according to claim 1 or 2, characterized in that step (d) can be performed after each step (a), (b) and (c) to record the value of the adjustment obtained at the corresponding step.
 4. Method according to any one of claims 1 to 3, characterized in that each step (a), (b) or (c) comprises, at the start thereof, a preliminary step of checking the stability of the image.
 5. Method according to claim 4, characterized in that the preliminary step of checking the stability of the image comprises the following operations: (a₀) carrying out (62, 64) pairs of measurements of the durations black edges of the image (T1_(HAV), T2_(HAV) and/or T1_(VAV), T2_(VAV)), (a₁) subtracting (66) two successive measurements to determine the variation (DIFF) in the durations of the black edges of the image, (a₂) comparing (68) that variation with a threshold (TMUDIFF), and in the case where said threshold is exceeded, stopping the process, in the opposite case, passing on to the next operation, (a₃) comparing (70) the measurements (T1_(HAV), T2_(HAV) and/or T1_(VAV), T2_(VAV)) with a threshold (MAX), and in case of that threshold being exceeded, stopping the process, in the opposite case, passing on to the next operation of the step (a), (b) or (c) in course of being executed.
 6. Method according to claim 5, characterized in that operation (a₀) only performs measurements relative to those concerned by step the (a), (b) or (c) in the course of being executed.
 7. Method according to any of claims 1 to 6, characterized in that step (a) further comprises the following operations: (a₄) checking (72) that the measurements (T1_(HAV), T2_(HAV)) are the first since the start of the process, in the case where the response is negative, passing on to sub-step (a₆), in the case where the response is positive, passing on to the next sub-step (a₅), (a₅) comparing (74) the measurements (T1_(HAV) and T2_(HAV)), and setting a flag to the logic value “0” (76) if T1_(HAV)<T2_(HAV) to indicate that the image must be displaced towards the right, setting the flag to the logic value “1” (78) if T1_(HAV)>T2_(HAV) to indicate that the image must be displaced towards the left, stopping the horizontal centering if T1_(HAV)=T2_(HAV), since the image is centered horizontally, and passing on to step (b), passing next to the following sub-step (a₆), (a₆) checking whether the flag is at logic “1” (80), and in the case of a positive response, passing on to the next sub-step (a₇), in the case of a negative response, passing on to sub-step (a₈), (a₇) checking whether T1_(HAV)>T2_(HAV) (82) and, stopping the process if T1_(HAV)=T2_(HAV) since the image is horizontally centered, in the case of a positive response, passing on to sub-step (a₉), and in the case of a negative response, passing on to sub-step (a₁₀), (a₈) checking whether T1_(HAV)<T2_(HAV) (84) and, stopping the horizontal centering if T1_(HAV)=T2_(HAV), since the image is horizontally centered and passing on to step (b), in the case of a positive response, passing on sub-step (a₁₁), and in the case of negative response, passing on to sub-step (a₁₂); (a₉) decrementing (86) by one unit the adjustment value for centering (HPOS) to displace the image by one incremental step to the left, and passing on to sub-step (a₁₃), (a₁₀) incrementing (94) by one unit the adjustment value for centering (HPOS) to displace the image towards the right, since the image had been displaced by one incremental step too far towards the left, and stopping the horizontal centering since the image is centered horizontally and passing on to step (b); (a₁₁) incrementing (88) by one unit the adjustment value for centering (HPOS) to displace the image by one incremental step to the right, and passing on to sub-step (a₁₃), (a₁₂) decrementing (96) by one unit the adjustment value for centering (HPOS) to displace the image towards the left, since the image had been displaced by one incremental step too far towards the right, and stopping the horizontal centering since the image is centered horizontally and passing on to step (b); (a₁₃) incrementing (90) a loop counter by one unit each time a sub-step (a₉) or (a₁₁) has been performed, then passing on to the following sub-step (a₁₄), (a₁₄) checking (92) whether the content of the loop counter has attained a threshold (BMAX), et in the case of a positive answer, stopping the process on account of an impossibility of adjustment, in the case of negative answer, restarting the loop at step (a₂).
 8. Method according to any one of claims 1 to 7, characterized in that step (b) further comprises the following operations: (b₁) measuring the durations of the vertical black edges of the image (T1_(HAV), T2_(HAV)), (b2) calculating the value of the adjustment for horizontal dimensioning (HSIZE) using the formula: ${HSIZE} = {\frac{{A_{Vopti}\left( {T^{\prime}/{Td}^{\quad \prime}} \right)} - H_{AMPMIN}}{H_{AMPMAX} - H_{AMPMIN}} \times {HSIZE}_{MAX}}$

 in which formula: A_(Vopti) is the optimum amplitude of the current in the horizontal deflection coil to obtain an image of optimum width; this amplitude is measured for a type of cathode ray tube, HSIZE_(MAX) is the maximum value of HSIZE, e.g. 255 as indicated above, H_(AMPMAX) is the maximum variation of the current in the horizontal deflection coil to obtain a maximum horizontal deflection; this value varies as a function of the horizontal scanning frequency f(fH) as described below; H_(AMPMIN) is the minimum variation of the current in the horizontal deflection coil to obtain a minimum horizontal deflection; this value varies as a function of the horizontal scanning frequency f(fH) as described below; T′ is the total duration of a horizontal line, i.e. the duration of the period T of the horizontal synchronization signal, from which are subtracted the duration of the flyback pulse T_(FLYBACK), in general three microseconds, and a duration of safety margins, e.g. 0.6 microseconds, and −Td′=T−(T1_(HAV) +T2_(HAV) +T _(FLYBACK))  i.e. the duration of the image on screen between these black edges.
 9. Method according to claim 8, characterized in that in the formula of claim 8, the coefficient (T′/Td′) is replaced by: 1.8T′−Td′/2.8Td′ to take into account the form of the curve for the current in the horizontal deflection coil.
 10. Method according to claim 8 or 9, wherein the values of H_(AMPMIN) and H_(AMPMAX) are determined using curves established for each range of frequencies.
 11. Method according to any one of claims 1 to 10, characterized in that step (c) further comprises the following operations: (c₁) measuring the durations of the horizontal black edges of the image (T1_(VAV), T2_(VAV)), (c₂) calculating the adjustment value for the vertical dimensioning VSIZE′ using the formula: VSIZE′=0.5[(3VSIZEMAX+2VSIZE)].[(Td×T′)/(Td′×T)]−1.5(VSIZEMAX), (C₃) calculating the adjustment value for the vertical centering VPOS′ using the formula: VPOS′=VPOS+(A−B)withA=[(2.25+1.5.(VSIZE/VSIZEMAX)]×[(0.5−T1/T).(VPOSMAX/0.6)]andB=[(2.25+1.5.(VSIZE′/VSIZEMAX)]×[0.5−T1/T′).(VPOSMAX/0.6)],  in which formula: Td is the duration of a reference image such that: Td=T2−T1 with T1=T1_(VAV) and T2=T−T2_(VAV), T is the total duration of a vertical scanning sawtooth, Td′ is the duration of the image to dimension and to center such that: Td′=T2′−T1′ with T1′=T1′_(VAV) and T2′=T′−T2′_(VAV), VSIZE is the adjustment value for the vertical dimension of the reference image, VSIZEMAX is the maximum adjustment value for vertical dimension of the image, VPOS is the adjustment for the vertical centering of the reference image, and VPOSMAX is the maximum adjustment value for vertical centering. 